Method of deciding on catalyst deterioration and means for deciding on catalyst deterioration in Nox purging system

ABSTRACT

The present invention provides a method of deciding on catalyst deterioration in an exhaust gas decontamination system, which method enables making an accurate decision on the state of deterioration of catalyst caused by sulfur poisoning in a NOx purging system using a direct reduction type NOx catalyst in the purging of NOx from exhaust gas. In particular, a method of deciding on catalyst deterioration in a NOx purging system ( 10 ) comprising exhaust gas passage ( 2 ) and, arranged therein, direct reduction type NOx catalyst ( 3 ), wherein when the operating condition of engine ( 1 ) is within a deterioration decision zone and is of stationary operation, an exhaust gas for decision is generated and wherein when the concentration of NOx in exhaust gas (Cnox) resulting from pass of the exhaust gas for decision through the direct reduction type NOx catalyst ( 3 ) is not below prescribed decision level (Cnoxlim), the direct reduction type NOx catalyst ( 3 ) is judged as being deteriorated.

TECHNICAL FIELD

The present invention relates to a method of deciding on catalystdeterioration and means for deciding on catalyst deterioration in NOxpurging system which is the system for reducing and purging NOx in theexhaust gas of an internal combustion engine or combustion equipment.

More minutely, the present invention relates to the method and means tobe able to make an accurate decision on the state of deterioration ofcatalyst caused by sulfur poisoning in a NOx purging system using adirect reduction type NOx catalyst for purging NOx from exhaust gas.

BACKGROUND ART

Various studies and proposals have been offered regarding acatalyst-type exhaust gas purging system for purging NOx (nitrogenoxides) from the exhaust gas of an internal combustion engine of avehicle or a stationary internal combustion engine by reducing NOx.Particularly, a NOx reduction catalyst or a three-way-catalyst is in useto purify the exhaust gas of vehicles.

One of the studies and proposals shows an exhaust gas purifying systemfor internal combustion engine disclosed in the official gazette of theJapanese Patent Laid-Open No. 2000-274279 and the like. In this system,a NOx occlusion reduction catalyst is arranged in an exhaust gas passageof an engine. This system performs the absorbing operation of making theNOx occlusion reduction catalyst absorb NOx during the air/fuel ratio ofan incoming exhaust gas is lean. When the absorbed quantity almostreaches to the level of NOx absorbing capacity, the regeneratingoperation is performed to make the air/fuel ratio of the exhaust gasclose to a theoretical air/fuel ratio or rich and to lower an oxygenconcentration of the incoming exhaust gas, and thereby releasing theabsorbed NOx, and reducing the released NOx by a laid-in precious metalcatalyst.

The NOx occlusion reduction catalyst supports a precious metal catalystsuch as platinum (Pt) and alkaline earth such as barium (Ba) on acatalyst support. NO in the exhaust gas is oxidized by a catalystactivity of platinum and changed to NO₂ under ahigh-oxygen-concentration atmosphere. NO₂ is diffused in the catalyst inthe form of NO₃ ⁻, and absorbed in the form of nitrate.

Moreover, when the air/fuel ratio becomes rich and the oxygenconcentration lowers, NO₃ ⁻ is released in the form of NO₂. And the NO₂is reduced to N₂ in accordance with the catalyst activity of platinum byreducer such as unburned HC, CO, and H₂ contained in the exhaust gas. Itis possible to prevent NOx from being released to the atmospheric air inaccordance with the above reducing effect.

However, in the case of the exhaust gas purging system using the NOxocclusion reduction catalyst, an extremely large quantity of NOx isreleased in a short time when the NOx occlusion reduction catalyst isregenerated. Therefore, it is necessary to reduce the NOx by a preciousmetal catalyst. However, even if supplying a proper quantity ofreducers, it is difficult to reduce the total quantity of NOx to N₂ bysecurely bringing the total quantity of NOx into contact with thereducers and the precious metal catalyst. Then, some of NOx leaks.Therefore, there is a problem that a decrease of NOx is limited.

Moreover, because a catalyst function is deteriorated by the sulfurcontained in the fuel of a diesel engine, there is a problem of sulfurpoisoning that it is difficult to keep the rate of NOx purge high for along time. Therefore, the exhaust gas purifying system in the officialgazette of the Japanese Paten Laid-Open No. 2000-274279 performs thedeterioration judgment that a NOx occlusion reduction catalyst isdeteriorated when the NOx concentration at the end of release of NOx isequal to or higher than a predetermined reference value for thedeterioration, in accordance with the characteristic of an occlusioncatalyst to store a large quantity of NOx by absorbing substance and torelease the NOx from the substance.

For the sulfur purge for reactivating the catalyst from the deterioratedstate due to the sulfur poisoning, it is necessary to raise the catalysttemperature up to 650° C. In the case of a diesel engine, it isnecessary to raise the exhaust gas temperature to 600° C. or higher inorder to raise the catalyst temperature to 650° C. or higher. However,even if performing the exhaust gas temperature raising control such asintake throttling or rich burning, it is actually difficult to raise thecatalyst temperature up to 650° C. by only controlling the engine.

However, separately from the NOx occlusion reduction catalyst, acatalyst for directly reducing NOx (hereafter referred to as directreduction type NOx catalyst) is disclosed in the patent applications toRepublic of Finland No. 19992481 and 20000617.

The direct reduction type NOx catalyst is obtained by making a catalystsupport T such as β-type zeolite support a metal M such as rhodium (Rh)or palladium (Pd) which is a catalyst component as shown in FIGS. 7 and8. As shown in FIG. 7, in the case of high oxygen concentrationatmosphere such as the lean state exhaust gas of an internal combustionengine such as a diesel engine in which the air/fuel ratio of exhaustgas is lean, the catalyst component contacts with NOx and reduces theNOx to N₂. At the same time, it is oxidized to become metal oxide MOxsuch as rhodium oxide. After all of the metal M is oxidized, thecapability of NOx reduction disappears. Therefore, it is necessary toregenerate the metal M when it is oxidized to a certain extent.

As shown in FIG. 8, the above regeneration is performed by setting theoxygen concentration of the exhaust gas to almost equal to 0% as theair/fuel ratio is in a theoretical air/fuel ratio or a rich state, bybringing the metal oxide MOx such as rhodium oxide into contact withreducer such as unburned HC, CO, and H₂ in a reduction atmosphere toreduce the metal oxide MOx, and by returning the metal oxide M to itsoriginal metal M.

In the case of the direct reduction type NOx catalyst, the reaction forreducing the metal oxide MOx is quickly performed even at a lowtemperature (e.g. 200° C. or higher) compared to the case of othercatalyst. And moreover, there is an advantage that the problem of sulfurpoisoning is small.

Moreover, cerium (Ce) is blended. This cerium contributes for decreasingthe oxidation of the metal M and for holding the capability of reductionof NOx. And a three-way-catalyst is set to a lower layer to acceleratethe reaction of reduction and oxidation, particularly the reaction ofreducing NOx in a rich state. Moreover, iron (Fe) is added to thecatalyst support in order to improve the rate of NOx purge.

However, though sulfur poisoning is small compared to the case of theNOx occlusion reduction catalyst, sulfur poisoning is slowly progressedby the sulfur in a fuel. Then the deterioration of the catalyst isprogressed. That is, because the sulfur contained in the exhaust gas isabsorbed as SO₂ in the iron added to the catalyst support, primarysulfur poisoning occurs in which the improvement of the purgeperformance of NOx due to the iron is inhibited. Moreover, in anoxidizing atmosphere containing no reducer at a constant temperature,SO₂ discharged from iron is changed to SO₃ and the SO₃ is combined withcerium. Therefore, the contribution of the cerium to holding thecapability of the NOx reduction is lowered and the rate of NOx isdecreased.

When the deterioration progresses in the direct reduction type NOxcatalyst, the rate of NOx purge is decreased because the capability ofreducing NOx to N₂ is decreased even in the case of an atmosphere inwhich the air/fuel ratio of the exhaust gas is in the lean state and theoxygen concentration is high. And moreover, the NOx reduction capabilityis immediately decreased to a value close to a limit. Then, because itis necessary to frequently perform regeneration by rich burning, fuelefficiency decreases.

Therefore, in the case of the direct reduction type NOx catalyst, it isnecessary to perform not only the regeneration of reducing the metaloxide MOx to the metal M by bringing into contact with reducers in areducing atmosphere, but also the reactivation of removing sulfur fromcatalyst by monitoring a progress state of deterioration due to thesulfur poisoning and by setting an exhaust gas temperature to approx.400° C. in a low oxygen concentration state when the deterioration isprogressed to a certain extent.

Moreover, in the case of the direct reduction type NOx catalyst, as acharacteristic of the NOx catalyst, it is found through an experimentthat the following phenomenon occurs at a low SV (Space Velocity) of50,000/h or less. Only in a state in which the NOx catalyst isdeteriorated due to sulfur poisoning, a large quantity of NOx isdischarged when a catalyst temperature is kept in a range of 250° C. to350° C. and the air/fuel ratio is lowered to approx. 23 during a steadyoperation.

SUMMARY OF THE INVENTION

The present invention is made to solve the above problems by obtainingthe above knowledge and its object is to provide a method means ofdeciding on catalyst deterioration and method for deciding on catalystdeterioration in an exhaust gas decontamination system, which enablesmaking an accurate decision on the state of deterioration of catalystcaused by sulfur poisoning in a NOx purging system using a directreduction type NOx catalyst to purge NOx from exhaust gas.

The method for achieving the above object is the method of deciding oncatalyst deterioration in the NOx purging system comprised of a directreduction type NOx catalyst arranged in an exhaust gas passage in whicha catalyst component reduces NOx to nitrogen and is also oxidized whenan oxygen concentration in the exhaust gas of an engine is high and thecatalyst component is reduced when the oxygen concentration in theexhaust gas is low, which comprises generating the exhaust gas fordecision when an operating state of an engine is within a deteriorationdecision zone and is in a steady operation state, and deciding that thedirect reduction type NOx catalyst is deteriorated when the NOxconcentration in the exhaust gas resulting from pass of the exhaust gasfor decision through the direct reduction type NOx catalyst is not lessthan a give reference value.

It is possible to constitute the direct reduction type NOx catalyst bymaking a catalyst support such as β-type zeolite support a special metalsuch as rhodium (Rh) or palladium (Pd) which is a catalyst component.Moreover, it is possible to form the direct reduction type NOx catalystby blending cerium (Ce) in order to decrease the oxidation action of ametal of the catalyst component and to contribute to holding thecapability of reduction of NOx. And it is possible to set athree-way-catalyst having platinum (Pt) or the like to a lower layer inorder to accelerate a redox reaction, particularly the reductionreaction of NOx discharged in a rich state, or to add iron (Fe) to thesupport in order to improve the rate of NOx purge.

A catalyst in which a catalyst component reduces NOx to N₂ and thecatalyst component is oxidized when the oxygen concentration in theexhaust gas is high, and the catalyst component is reduced when theoxygen concentration of the exhaust gas is decreased, is referred to as“direct reduction type NOx catalyst” in this case in order todistinguish this catalyst from catalysts used in other prior arts.

Moreover, a prescribed reference value for a NOx concentration is anumerical value or map data obtained through an experiment, which is apreset value.

Furthermore, in the case of the method of deciding on catalystdeterioration in a NOx purging system, it is decided that the operatingstate of the engine is within a deterioration decision zone, when thequantity of the exhaust gas is not more than a prescribed referencequantity of exhaust gas and a catalyst temperature ranges between aprescribed lower limit temperature for decision and a prescribed upperlimit temperature for decision.

The prescribed reference quantity of exhaust gas is an upper limitquantity of the exhaust gas when a space velocity (SV) for a directreduction type NOx catalyst is not more than 50,000/h and values of 250°C. and 350° C. are experimentally obtained as a prescribed lower limittemperature for decision and a prescribed upper limit temperature fordecision.

Moreover, the decision that an operation state is in a steady operationsate, can be performed by judging that the present state is the steadyoperating state when the absolute value of a change value ΔQ of a torqueQ is not more than a prescribed reference value ΔQlim and the absolutevalue of a change value ΔNe of an engine speed Ne is not more than aprescribed reference value ΔNelim.

Furthermore, in the case of the method of deciding on catalystdeterioration in a NOx purging system, the exhaust gas for decision isgenerated so that the air/fuel ratio of exhaust gas becomes a valuebetween the air/fuel ratio of the exhaust gas during the normal engineoperation and the air/fuel ratio of the exhaust gas for regenerating thedirect reduction type NOx catalyst.

The air/fuel ratio of the exhaust gas for decision is approx. 23,preferably 22 to 25, when the base of the air/fuel ratio is approx. 27and the air/fuel ratio of regeneration exhaust gas is 14.7 or less. Theexhaust gas for decision can be produced by any one of a fuel injectioncontrol, an intake air quantity control, and an EGR control or acombination of them.

The above normal engine operation is not a control operation forregenerating a catalyst or reactivating a deteriorated catalyst but anoperation for operating an engine at a torque and a engine speedrequested to the engine. In the case of the normal engine operation, theair/fuel ratio of the exhaust gas is approx. 27 in a deteriorationdecision zone and NOx contained in the exhaust gas is directly reducedto N₂ and purged by a direct reduction type NOx catalyst.

Moreover, the means for deciding on catalyst deterioration in a NOxpurging system for executing the above method of deciding on catalystdeterioration is the means of deciding on catalyst deterioration in aNOx purging system composed of a direct reduction type NOx catalystarranged in an exhaust gas passage in which a catalyst component reducesNOx to nitrogen and is also oxidized when an oxygen concentration in theexhaust gas of an engine is high and the catalyst component is reducedwhen the oxygen concentration in the exhaust gas is low, which comprisesa zone judgment means for judging whether an exhaust gas state is in azone capable of performing a decision for the catalyst deterioration, asteady operation judgment means for judging whether an engine operatingstate is in a steady operation state, a decision exhaust gas generationmeans for generating an exhaust gas for decision, and a NOxconcentration judgment means for deciding that the direct reduction typeNOx catalyst is deteriorated when the NOx concentration in the exhaustgas resulting from pass of the exhaust gas for decision through thedirect reduction type NOx catalyst is higher than a prescribed referencevalue.

Moreover, in the case of the above means for deciding on catalystdeterioration in a NOx purging system, the zone judgment means decidesthat the operating state of the engine is within a deteriorationdecision zone, when the quantity of the exhaust gas is not more than aprescribed reference quantity of the exhaust gas and a catalysttemperature ranges between a prescribed lower limit temperature fordecision and a prescribed upper limit temperature for decision.

Furthermore, in the case of the above means for deciding on catalystdeterioration in a NOx purging system, the decision exhaust gasgeneration means generates the exhaust gas for decision in which anair/fuel ratio of the exhaust gas becomes a value between the air/fuelration of the exhaust gas during the normal engine operation and theair/fuel ratio of the exhaust gas for regenerating the direct reductiontype NOx catalyst.

According to the method of deciding on catalyst deterioration and meansfor deciding on catalyst deterioration in a NOx purging system havingthe above configuration, because a deteriorated state of a catalyst dueto sulfur poisoning can be accurately decided in a NOx purging systemusing the direct reduction type NOx catalyst to purge NOx in the exhaustgas, it is possible to perform the operation to reactivate thedeteriorated catalyst against the catalyst deterioration in a propertime.

Therefore, it is possible to reduce the influence of the catalystdeterioration relating to the time for NOx reduction and regeneration orthe like in the direct reduction type NOx catalyst and to purgeefficiently the NOx at a high purging performance. Thus, it is possibleto further reduce the exhaust quantity of NOx and further improve a NOxpurging performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a configuration of a NOx purgingsystem of an embodiment of the present invention;

FIG. 2 is an illustration showing a configuration of a NOx purgingsystem control means of an embodiment of the present invention;

FIG. 3 is a flowchart showing an example a NOx purging system controlflow of an embodiment of the present invention;

FIG. 4 is a flowchart showing an example of the catalyst regeneratingcontrol flow in FIG. 3;

FIG. 5 is a flowchart showing an example of the deteriorated catalystreactivation control flow in FIG. 3;

FIG. 6 is a flowchart showing an example of the determination judgmentflow of an embodiment of the present invention;

FIG. 7 is a schematic view showing a reaction of a direct reduction typeNOx catalyst at a high oxygen concentration; and

FIG. 8 is a schematic view showing a reaction of a direct reduction typeNOx catalyst in a low oxygen concentration state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of the method of deciding on catalyst deterioration andmeans for deciding on catalyst deterioration in a NOx purging systemaccording to the present invention are described below by referring tothe accompanying drawings.

First, the NOx purging system in which the method of deciding oncatalyst deterioration and means for deciding on catalyst deteriorationare used is described below.

As shown in FIG. 1, the NOx purging system 10 is comprised of a directreduction type NOx catalyst 3 arranged in an exhaust gas passage(exhaust passage) 2 of an engine body 1.

As shown in FIGS. 7 and 8, the direct reduction type NOx catalyst 3 iscomposed by making a catalyst support T such as β-type zeolite support aspecial metal M such as rhodium (Rh) or palladium (Pd). Moreover, cerium(Ce) is bended for reducing an oxidization of the metal M andcontributing to holding of a NOx reduction capability, athree-way-catalyst having platinum (Pt) or the like is arranged to alower layer so as to accelerate a redox reaction, and iron (Fe) is addedto the support in order to improve a rate of NOx purge.

Then, as shown in FIG. 7, the direct reduction type NOx catalyst 3reduces NOx to N₂ contacting with NOx and the metal M itself is oxidizedto become metal oxide MOx such as rhodium oxide (RhOx), in an atmosphereof a high oxygen concentration like an exhaust gas of an internalcombustion engine such as a diesel engine in which the air/fuel ratio islean. As shown in FIG. 8, however, the metal oxide MOx has acharacteristics to be reduced to become its original metal M such asrhodium (Rh) by contacting with reducers such as unburned HC, CO, andH₂, in the case of a reduction atmosphere in which an oxygenconcentration of the exhaust gas is almost equal to 0% like a case asthe air/fuel ratio is equal to a theoretical air/fuel ratio or in a richstate.

Moreover, an operating state detector 5 is set which is comprised of atorque sensor and an engine speed sensor for detecting the operatingstate of an engine, mainly a torque Q and an engine speed Ne.Furthermore, an air/fuel ratio sensor 6 for detecting an air/fuel ratioAf is set upstream of the direct reduction type NOx catalyst 3 arrangedin the exhaust gas passage 2. And a catalyst temperature sensor 7 fordetecting a catalyst temperature Tcat is set in the direct reductiontype NOx catalyst 3, and moreover a NOx sensor 8 for detecting a NOxconcentration Cnox is set downstream of the NOx catalyst 3.

Then, a controller 4 referred to as an engine control unit (ECU) forperforming the general control of an engine such as fuel injectioncontrol by using the torque (load) Q and engine speed Ne of the engine 1obtained from the operating state detector 5 or the like as inputs isprovided. And a NOx purging system control means for performing thecatalyst regeneration control and the deteriorated catalyst reactivationcontrol for the direct reduction type NOx catalyst 3 is set in thecontroller 4.

As shown in FIG. 2, a NOx purging system control means 200 is composedof a catalyst regeneration means 210 and a deteriorated catalystreactivation means 220. The catalyst regeneration means 210 is composedof a regeneration time judgment means 211 and a regeneration controlmeans 212, and the deteriorated catalyst reactivation means 220 iscomposed of a deterioration judgment means 221 and a reactivationcontrol means 222.

The catalyst regeneration means 210 is a means for regenerating thedirect reduction type NOx catalyst 3 in which the metal M has changedthe metal oxide MOx by contacting with NOx to redue NOx to N₂ in thenormal operating state of high oxygen concentration where the air/fuelratio of the exhaust gas is in a lean state.

The regeneration time judgment means 211 judges the time for performingthe regeneration. When it judges that it is the time, the regenerationcontrol means 212 generates the exhaust gas of theoretical air/fuelratio or a rich state in which the oxygen concentration is almost equalto 0%, and makes the metal oxide MOx contact with reducers such asunburned HC, CO, and H₂ to reduce the metal oxide MOx and to return itto the metal M.

The regeneration time judgment means 211 judges whether it is theregeneration time or not by the NOx concentration Cnox of the exhaustgas downstream of the direct reduction type NOx catalyst 3 when reducingNOx, by the elapsed time during which the oxygen concentration is high,and by the estimated value of the quantity of NOx reduced by the directreduction type NOx catalyst 3 when reducing NOx.

Moreover, the regeneration control means 212 is a means for decreasingthe oxygen concentration of the exhaust gas, that is, a means forperforming the rich spike operation with the air/fuel ratio Af of 14.7or less. This means 212 performs any one or a combination of thecontrols such as a fuel injection control for controlling the injectionof the fuel to be supplied to the combustion chamber of an internalcombustion engine, an intake air control for controlling the quantity ofintake air, and an EGR control for controlling the quantity of EGR gasin an EGR system, and performs a feedback control so that the detectionvalue Af is kept within a prescribed set range in accordance with thedetection value Af of the air/fuel ratio sensor 6.

The fuel injection control includes a main injection time control forchanging time of the main fuel injection into the combustion chamber ofan engine and a post-injection control for performing a post-injectionafter a main injection. And the intake air control includes an intakethrottle valve control for controlling a valve opening of anot-illustrated intake throttle valve and a turbocharger intake aircontrol for controlling the quantity of an intake air from a compressorof a not-illustrated turbocharger.

Moreover, the deterioration judgment means 221 of the deterioratedcatalyst reactivation means 220 relates to the present invention, whichis a means for judging the deteriorated state of the direct reductiontype NOx catalyst 3 as the decision whether on a reactivation time ornot. And this means 221 is composed of a deterioration zone judgmentmeans 221 a, a steady operation judgment means 221 b, a decision exhaustgas generation means 221 c, and a NOx concentration judgment means 221d.

The deterioration zone judgment means 221 a is a means for judgingwhether the exhaust gas state is in the zone in which it is capable ofperforming the decision on catalyst deterioration. This means 221 ajudges that the exhaust gas state is in the deterioration decision zonewhen the quantity Qe of the exhaust gas is not more than a prescribedreference quantity Qelim of exhaust gas and a catalyst temperature Tcatranges between a prescribed lower limit temperature TL for decision anda prescribed upper limit temperature TH for decision.

The prescribed reference quantity Qelim of exhaust gas is previously setas a value at which the value of a space velocity (SV) to the directreduction type NOx catalyst 3 becomes a low SV state of 50,000/h orless. Moreover, from experimental results, 250° C. is obtained as theprescribed lower limit temperature TL for decision and 350° C. isobtained as the prescribed upper limit temperature TH for decision.

Moreover, the value of the SV is a value obtained by dividing an exhaustgas flow rate by the volume of a catalyst system and serving as apassing velocity.

The steady operation judgment means 221 b is a means for judging whetheran engine operating state is of stationary state of an engine. Thismeans 221 b judges that the present operation is the steady operationwhen the absolute value of a change value ΔQ of the torque Q is not morethan the prescribed reference value ΔQlim and the absolute value of achange value ΔNe of the engine speed Ne is not more than the prescribedreference value ΔNelim.

The decision exhaust gas generation means 221 c is a means forgenerating an exhaust gas for decision in which the air/fuel ratio Af isapprox. 23. This means 211 c performs feedback control in accordancewith the value of the air/fuel ratio Af detected by the air/fuel ratiodetector 6 to generate the exhaust gas having a prescribed air/fuelratio. The exhaust gas is generated in accordance with any one of a fuelinjection control, an intake air control, and an EGR control or acombination of them.

Moreover, the NOx concentration judgment means 221 d judges that thedirect reduction type NOx catalyst 3 is deteriorated when the NOxconcentration Cnox where a state of exhaust gas is in state of theexhaust gas for decision is larger than a prescribed reference valueCnoxlim and returns by setting a deterioration judgment flag F2 to 1.However, when the NOx concentration Cnox is smaller than the prescribedreference value Cnoxlim, the means 221 d judges that the catalyst 3 isnot deteriorated and returns by setting the deterioration judgment flagF2 to 0.

Furthermore, the reactivation control means 222 of the deterioratedcatalyst reactivation means 220 is a means for reactivating the directreduction type NOx catalyst 3 deteriorated due to sulfur poisoning. Thisreactivation is carried by sulfur-purge. This means 222 performs thecontrol for raising the catalyst temperature Tcat to 400° C. or higherwhile bringing an oxygen concentration of the exhaust gas to a valueclose to 0%.

Then, a NOx purging system control flow is described below in which NOxis purged from the exhaust gas by controlling the NOx purging system 10of above configuration by the NOx purging system control means 200. Thecontrol flow is performed in accordance with the flowcharts and the likeillustrated in FIGS. 3 to 6.

The NOx purging system control flows illustrated in FIGS. 3 to 6 aredesigned as a part of the general flow for generally controlling anengine, which is called from a main engine control flow and executed inparallel with an engine control flow. After this control flow isexecuted, the general flow repeatedly returns to the main engine controlflow and is completed in accordance with the completion of the enginecontrol flow.

In the case of the NOx purging system control flow in FIG. 3, the normaloperation control for purging NOx by the direct reduction type NOxcatalyst 3 is performed for a prescribed period (for example, the timeperiod for judging whether to perform the catalyst regeneration controlor the deteriorated catalyst reactivation control) in step S100. And, instep S200, as shown by the catalyst regeneration control flow in FIG. 4,it is judged whether the direct reduction type NOx catalyst 3 is in aregeneration time in step S210. Through this checking in step S220, theregeneration control in step S230 is executed when the present time isthe regeneration time (F1=1). But, when the present time is not theregeneration time (F1=0), the flow returns directly and goes to thedeteriorated catalyst reactivation control in the next step S300 in FIG.3.

In the case of the deteriorated catalyst reactivation control, as shownin the deteriorated catalyst reactivation control flow in FIG. 5, it isjudged whether the direct reduction type NOx catalyst 3 is deterioratedin step S310. Through this checking in step S320, when it is judged thatthe catalyst 3 is deteriorated (F2=1), the deteriorated catalystreactivation control is executed in step S330. However, when it isjudged that the catalyst 3 is not deteriorated (F2=0), the flow returnsdirectly to the NOx purging system control flow in FIG. 3.

Then, the flow goes to the return at the NOx purging system control flowin FIG. 3 to return to a not-illustrated main engine control flow andthe NOx purging system control flow is repeated by being called againfrom the engine control flow.

Moreover, in the case of the exhaust gas purging system 10 of thepresent invention, the deterioration judgment for the direct reductiontype NOx catalyst 3 (step S310) is performed by the method of decidingon catalyst deterioration based on the deterioration judgment flowillustrated in FIG. 6.

When the deterioration judgment flow in FIG. 6 starts, the detection andcontrol values showing operation state such as the torque Q and enginespeed Ne are first read from the operating state detector 5 in stepS311. Then, in step S312, it is judged whether the quantity Qe of theexhaust gas is not more than the prescribed reference quantity Qelim ofexhaust gas at which the present state becomes a low SV state in whichthe value of the space velocity (SV) to the direct reduction type NOxcatalyst 3 is 50,000/h or less and the catalyst temperature Tcat rangesbetween the prescribed lower limit temperature TL (250° C.) for decisionand the prescribed lower limit temperature TH (350° C.) for decision.

When it is judged in step S312 that an operating state is not in thedeterioration decision zone, the flow returns. However, when it isjudged that the operating state is in the deterioration decision zone,it is judged in step S313 whether the operating state is in a steadyoperation state in accordance with whether the absolute value of thechange value ΔQ of the torque Q is not more than the prescribedreference value ΔQlim and the absolute value of the change value ΔNe ofthe engine speed Ne is not more than the prescribed reference valueΔNelim.

When it is judged that a stationary operation is not currentlyperformed, in step S313, the flow returns. However, when it is judgedthat the stationary operation is currently performed, the exhaust gasfor decision in which the air/fuel ratio is approx. 23 is generated instep S314. And it reads the information of the exhaust gas on valuesdetected by the air/fuel ratio sensor 6 and the NOx sensor 8 in nextstep S315.

In step S316, it is checked whether the air/fuel ratio Af detected bythe air/fuel ratio sensor 6 is in a prescribed air/fuel range, that is,the ratio Af is a value approx. 23 of the exhaust gas for decision. WhenAf is not in the range, the flow returns to step S314 to wait until theAf falls into a prescribed range. When Af falls into the prescribedrange, the flow goes to step S317.

In step S317, it is checked whether the NOx concentration Cnox detectedby the NOx sensor 8 is higher than the prescribed reference valueCnoxlim. The prescribed reference value Cnoxlim is the data (numericalvalue or map data) obtained from an experiment, which is previouslyinput to the controller 4.

In step S317, it is judged that the direct reduction type NOx catalyst 3is deteriorated when the detected NOx concentration Cnox is higher thanthe reference value Cnoxlim to set the deterioration judgment flag F2 to1 and the flow returns. However, when lower than the reference valueCnoxlim, it is judged that the catalyst 3 is not deteriorated so set thedeterioration judgment flag F2 to 0 and the flow returns.

Then, in the deterioration state checking in step S320 in FIG. 5returned from the deterioration judgment flow, the deteriorationjudgment flag F2 is determined. When F2=1, the flow goes to thedeteriorated catalyst reactivation control in step S330. When F2=0, theflow returns to the NOx purging system control flow in FIG. 3.

According to the method of deciding on catalyst deterioration and themeans 221 for deciding on catalyst deterioration in the exhaust gaspurging system 10 having the above configuration, it is possible tocomparatively easily and accurately decide a deteriorated state or notby using the characteristic of a direct reduction type NOx catalyst whenit is deteriorated.

INDUSTRIAL APPLICABILITY

The present invention provides the method of deciding on catalystdeterioration and means for deciding on catalyst deterioration in anexhaust gas purging system capable of properly deciding a deterioratedstate of a catalyst due to sulfur poisoning in a NOx purging systemusing a direct reduction type NOx catalyst to purge the NOx contained inthe exhaust gas.

Therefore, it is possible to apply the present invention to NOx purgingsystems respectively using a direct reduction type NOx catalyst to purgethe NOx contained in the exhaust gas and to purify efficiently exhaustgas from internal combustion engines and stationary internal combustionengines of vehicles mounting these NOx purging systems.

1. A method of deciding on catalyst deterioration in a NOx purgingsystem comprised of a direct reduction type NOx catalyst arranged in anexhaust gas passage in which a catalyst component reduces NOx tonitrogen and is also oxidized when an oxygen concentration in theexhaust gas of an engine is high and the catalyst component is reducedwhen the oxygen concentration in the exhaust gas is low, whichcomprises; generating the exhaust gas for decision when an operatingstate of an engine is within a deterioration decision zone and is in asteady operation state, and deciding that the direct reduction type NOxcatalyst is deteriorated when the NOx concentration in the exhaust gasresulting from pass of the exhaust gas for decision through the directreduction type NOx catalyst is not less than a prescribed referencevalue.
 2. The method of deciding on catalyst deterioration in the NOxpurging system according to claim 1, which comprises; deciding that theoperating state of the engine is within a deterioration decision zone,when the quantity of the exhaust gas is not more than a prescribedreference quantity of exhaust gas and a catalyst temperature rangesbetween a prescribed lower limit temperature for decision and aprescribed upper limit temperature for decision.
 3. The method ofdeciding on catalyst deterioration in the NOx purging system accordingto claim 1, which comprises; generating the exhaust gas for decision sothat an air/fuel ratio of the exhaust gas becomes a value between theair/fuel ratio of the exhaust gas during the normal engine operation andthe air/fuel ratio of the exhaust gas for regenerating the directreduction type NOx catalyst.
 4. A means for deciding on catalystdeterioration in a NOx purging system comprised of a direct reductiontype NOx catalyst arranged in an exhaust gas passage in which a catalystcomponent reduces NOx to nitrogen and is also oxidized when an oxygenconcentration in the exhaust gas of an engine is high and the catalystcomponent is reduced when the oxygen concentration in the exhaust gas islow, which comprises; a zone judgment means for judging whether anexhaust gas state is in a zone capable of performing a decision for thecatalyst deterioration, a steady operation judgment means for judgingwhether an engine operating state is in a steady operation state, adecision exhaust gas generation means for generating an exhaust gas fordecision, and a NOx concentration judgment means for deciding that thedirect reduction type NOx catalyst is deteriorated when the NOxconcentration in the exhaust gas resulting from pass of the exhaust gasfor decision through the direct reduction type NOx catalyst is higherthan a prescribed reference value.
 5. The means for deciding on catalystdeterioration in a NOx purging system according to claim 4, wherein; thezone judgment means decides that the operating state of the engine iswithin a deterioration decision zone, when the quantity of the exhaustgas is not more than a prescribed reference quantity of the exhaust gasand a catalyst temperature ranges between a prescribed lower limittemperature for decision and a prescribed upper limit temperature fordecision.
 6. The means for deciding on catalyst deterioration in a NOxpurging system according to claim 4, wherein; the decision exhaust gasgeneration means generates the exhaust gas for decision in which anair/fuel ratio of the exhaust gas becomes a value between the air/fuelration of the exhaust gas during the normal engine operation and theair/fuel ratio of the exhaust gas for regenerating the direct reductiontype NOx catalyst.